In a cellular communication system, a communication link is established between a mobile station, or subscriber, and a source base station. As the mobile station moves out of range of the source base station, the signal quality will degrade until the communication link would ultimately be broken and the call "dropped". At this point, the communication link is shifted from the source base station to a target base station. The process of making this shift is commonly referred to in the cellular communication area as a handoff process.
Presently, handoffs are performed, generally, in two different ways. In analog type systems, such as the analog mobile phone system (AMPS), a handoff is accomplished by the system without assistance from the mobile station. When the signal received by the source base station degrades past a certain point, the source base station sends a message to the mobile switching center (MSC), by way of a base station controller (BSC), requesting that a handoff be conducted. Upon receipt of the request, the MSC sends messages to candidate base stations requesting that each measure a parameter, generally signal strength, of a signal transmitted by the mobile station. The candidate base stations make the measurement and return the results to the MSC. The MSC then makes the determination as to which candidate base station will be selected as the handoff, or target, base station. The MSC then orders the necessary connections to the handoff base station and directs the target base station to become the source base station. At the same time, the source base station is directed to discontinue serving the mobile station.
The above is the traditional type of handoff. In newer systems--such as time division multiple access (TDMA) and code division multiple access (CDMA) systems--a mobile assisted handoff (MAHO) process is utilized. In a MAHO type of system, the mobile station is provided with a list of candidate base stations. At intermittent times, the mobile station will measure a signal quality parameter of transmissions from the listed candidate base stations. The quality parameter may be signal strength, or other appropriate parameter; such as energy per chip per total noise (E.sub.C /I.sub.O), bit error rate (BER), frame erasure rate (FER), or color code. These measurements are gathered to determine a preferred list of target base stations and; when directed, the preferred list is reported to the MSC, by way of base station and BSC. The MSC then takes the same action as previously in selecting a handoff base station and directing the handoff.
The above processes are the two basic types of handoff. There are a number of variations within these basic types. For example, there is "soft" handoff. This occurs when the handoff base station starts serving the mobile station before the source base station discontinues service. There is also a softer handoff which is a soft handoff between two sectors within the same cell. In addition, the term "handover" is often substituted for the term "handoff" in certain geographic areas such as Europe.
Because of the difference in operation between the two handoff schemes, an AMPS base station, and BSC, cannot be coupled to a TDMA MSC and vice-versa. This is a disadvantage when a service provider decides to upgrade a system using a different air interface technology since the new technology would also require an additional MSC. Therefore, there is a need in the industry to provide a standard interface between a BSC and MSC which would permit a single MSC to operate with either an AMPS or TDMA/CDMA system without requiring an operating change in either of those systems.